The present invention relates to a method and apparatus for producing a coal/water mixture for combustion in a fluidized bed combustion unit, especially in a pressurized fluidized bed unit.
The prospectus "Das Heizkraftwerk der RWTH Aachen", January 1987, especially the inside of the cover page, FIG. 1a, and page 13, FIG. 1b, discloses a method of this type, where coal, accompanied by the addition of water and lime as a sulfur-binding agent, is preliminarily mixed and is final mixed in a mixer. The thus-produced mixture is supplied via a dense or heavy material pump to a fluidized bed combustion unit. The planned-for coal, with a grain-size fraction of 22% by weight &lt;1 mm (as addressed on page 4), could not always be delivered.
In the supply state, the coal is a crushed crude or raw coal. During the preliminary and final mixing, a fine grain-size fraction is rubbed off from the crushed raw coal. The hydraulic transportability of coal/water suspensions is known, in particular with combustion units that are fired with burners. The coal/water suspensions that are utilized in this connection are characterized by a very large fraction of fine coal particles, a relatively small maximum grain size of, for example, 2 mm, and a large proportion of water. However, for the requirements of the fluidized bed combustion units, the grain diameters of the suspensions that are suitable for the burners are too small. Especially for fluidized bed combustion units, considerably coarser grain sizes are required in order to be able to optimize the retention time of the coal in the combustion chamber, the combustion, and the flue gas emission. Furthermore, a water proportion that is too great reduces the calorific value of the fuel, and hence the efficiency of the combustion unit.
At the same time, where the coal/water mixture is hydraulically conveyed with pumps, especially heavy material pumps, the material being conveyed is compacted by the pressure. In other words, the gaps between grains are reduced, and the water that is bound to the fine grain-size fraction is pressed along with the latter against the walls of the conduits that are provided for conveying the coal/water mixture. This fine grain-size fraction, along with the water, form a lubricating film along the sliding contact surfaces of the conduits. Without this mechanism, it would not be possible to carry out a conveyance through the required lengths of conduits. Thus, when preliminarily crushed raw coal having little water added thereto is hydraulically conveyed via heavy material pumps, preferably piston pumps, and conduits, considerable difficulties arise. Thus, one must proceed from the assumption that the water requirement of raw coal, in order to form a mixture that can be conveyed by pumps, is a function of the grain composition and the gaps or spaces between the coal granules. At least for the binding, in an appropriate manner, of that water that is necessary for the conveyance, finely pulverized fine grain-size fractions in the granule spectrum are necessary. It furthermore appears that a certain percentage of medium-sized granules are advantageous in order to keep the gaps or spaces in the raw material small right from the beginning.
It is therefore an object of the present invention, proceeding from the aforementioned cited art, to provide a method and apparatus for producing a coal/water mixture where the hydraulic conveyance via the pumps is assured without at the same time permitting the proportion of water in the coal/water mixture to become too great.